1. Field of the Invention
The present invention relates to a thermistor device, further specifically, a thermistor device detectable of relatively high temperature.
2. Description of the Related Art
The mainstream of conventional thermistor devices for measuring the temperature of exhaust fume of an automobile, etc. has been those able to detect temperature up to 800° C. However recently, because of the increased demands for measuring the temperature of exhaust fume, etc. closer to an engine, it is desired to develop a thermistor device measurable of high temperature up to 1000° C.
As a high temperature heat-resistant type thermistor device, for example, as shown in Patent Document 1 (Japanese Unexamined Patent Publication No. 2007-180523), a thermistor device with improved heat-resistance is developed by devising a coating material covering around a main body of a device. However, the thermistor device shown in the Patent Document 1 is a single plate thermistor device without internal electrode layers, causing a problem that a surface of the device main body constituting a sensor portion is deteriorated by high heat to reduce sensor property.
Consequently, it is proposed that a laminate-type thermistor device is used for high-temperature applications, in which a surface of a device main body does not constitute a sensor portion, and an internal electrode layer and a thermistor layer are laminated. However, in conventional laminate-type thermistor devices, as shown in Patent Document 2 (the Japanese Unexamined Patent Publication No. 2007-180523), lead terminals are connected to both ends of the longest side in a device main body to result in an increase in distance between the lead terminals for the purpose of making an area of a laminated internal conductor layer as large as possible, etc.
When using such a conventional laminate-type thermistor device as a thermistor device for high-temperature applications at around 1000° C. without modification, the displacement of the device main body between the lead terminals may be increased due to heat expansion, and a crack may easily be caused in a device main body portion located at the center of the lead terminals (or insulating coating portion).
Also, in a laminate-type thermistor device in which a securing member is mounted in a lead terminal projecting from a device main body, the difference in heat expansion or heat contraction between the securing member and the device main body is increased, and it may cause stress in a direction to tear down the lead terminal from the device main body.